A dual stage model of anomalous moisture diffusion and desorption in epoxy mold compounds

Absorption and desorption tests were conducted on five distinct commercial epoxy mold compounds (EMCs) used in electronic packaging. For absorption, the samples were subjected to 85°C /85% relative humidity and 60°C /85% relative humidity soaking. Desorption conditions were above glass transition temperature at 140°C and 160°C. A dual stage model is developed in this paper for both absorption and desorption processes. Both stages in moisture absorption and desorption, i.e., Fickian diffusion and relaxation process, are described mathematically using a combination of Fickian terms. The models generated reasonable results for the diffusive properties and displayed outstanding experimental fits. All five compounds have shown strong non-Fickian diffusion behaviors, which were further demonstrated by experiments with different thicknesses. For absorption, results show Fickian diffusion is significantly faster than non-Fickian diffusion. Saturated moisture concentration associated with Fickian-stage diffusion is independent of temperature if it is below glass transition temperature. Sample thickness played a major role in diffusive behavior in the second stage where non-Fickian diffusion occurs. For desorption, higher temperature corresponds to less percentage of the permanent residual moisture content. At 160°C, 90% of the initial moisture for all samples could be diffused out within 24 hours, following a modified Fickian diffusion process. The dual stage model developed in this paper provides a foundation for modeling anomalous moisture diffusion behavior using commercial finite elemental method software.